When I open my show, the notice comes that a sound file (Slide 21.ogg) is missing and do I want to search for it, and if I don't the show may not play properly. The file is said to be associated with slide 19. There is no longer any such sound file. No sound file appears on Slide 19, but the notice at the top of Slide 19 says that Slide 21.ogg is missing. How do I get rid of these notices?